Pupillometry registers toddlers’ sensitivity to degrees of mispronunciation.

This study introduces a method ideally suited for investigating toddlers’ ability to detect mispronunciations in lexical representations: pupillometry. Previous research has established that the magnitude of pupil dilation reflects differing levels of cognitive effort. Building on those findings, we use pupil dilation to study the level of detail encoded in lexical representations with 30-month-old children whose lexicons allow for a featurally balanced stimulus set. In each trial, we present a picture followed by a corresponding auditory label. By systematically manipulating the number of feature changes in the onset of the label (e.g., baby ∼ daby ∼ faby ∼ shaby), we tested whether featural distance predicts the degree of pupil dilation. Our findings support the existence of a relationship between featural distance and pupil dilation. First, mispronounced words are associated with a larger degree of dilation than correct forms. Second, words that deviate more from the correct form are related to a larger dilation than words that deviate less. This pattern indicates that toddlers are sensitive to the degree of mispronunciation, and as such it corroborates previous work that found word recognition modulated by sub-segmental detail and by the degree of mismatch. Thus, we establish that pupillometry provides a viable alternative to paradigms that require overt behavioral response in increasing our understanding of the development of lexical representations.